Various compositions for the magnetic layer, various structures for the magnetic layer, and various materials for the nonmagnetic underlayer have been proposed to obtain a magnetic recording medium exhibiting a high recording density. One of the magnetic layers used in practice employs a CoCr alloy and obtains isolated magnetic grains by segregating Cr to the crystal grain boundary. Another one of the magnetic layers used in practice is the so-called granular magnetic layer that includes a nonmagnetic and nonmetallic material for the grain boundary thereof. For segregating sufficient amount of Cr into the grain boundary, it is necessary and indispensable to heat the substrate at 200° C. or higher during forming the conventional CoCr alloy magnetic layer. In contrast, the specific feature of the granular magnetic layer is that the nonmagnetic and nonmetallic material segregates even when the substrate is not heated during forming the granular magnetic layer. For realizing a higher recording density, research and development have been explored vigorously on the perpendicular magnetic recording that orients the recording magnetization perpendicular to the recording plane of the medium in place of the parallel magnetic recording that orients the recording magnetization parallel to the recording plane of the medium. The CoCr magnetic layer and the granular magnetic layer described above are employable for the perpendicular magnetic recording by controlling the crystal orientation therein by means of an underlayer.
It is required for both the CoCr magnetic layer and the granular magnetic layer to be thermally stable and cause less media noises. For improving the thermal stability, it is necessary to improve the crystalline magnetic anisotropy Ku. For reducing the media noises, it is necessary to minimize the crystal grain diameter in the magnetic recording layer and to reduce the magnetic interaction between the crystal grains. The crystalline magnetic anisotropy Ku in the CoCr magnetic layer is improved by adding an appropriate amount of Pt thereto. The magnetic interaction between the ferromagnetic crystal grains in the CoCr magnetic layer is reduced by promoting Cr segregation into the grain boundary by means of heating the substrate before forming the CoCr magnetic layer or by means of adding Ta or B to the CoCr magnetic layer. The other technique for segregation promotion, as reported in Journal of Applied Physics, Vol. 87, No. 9. pp. 6869-6871 (May 1, 2000), effectively isolates ferromagnetic crystal grains by depositing an Mn layer of 20 nm in thickness on a CoCrPt magnetic layer and by annealing the laminate at 350° C. for several minutes to diffuse Mn into the grain boundary of the CoCrPt magnetic layer.
For reducing the media noises and for realizing a high magnetic recording density, it is necessary to minimize the grain diameter in the magnetic recording layer and to magnetically isolate the crystal grains in the magnetic recording layer without impairing the thermal stability. However, the productivity of the above described segregation promotion technique that deposits an Mn layer on a CoCrPt layer and anneals the laminate is not that good since it takes several minutes for Mn to sufficiently diffuse into the grain boundary of the CoCrPt layer. Moreover, it is difficult to obtain a large signal output for the segregation promotion technique that deposits an Mn layer on a CoCrPt layer since the spacing between the magnetic head and the magnetic recording layer is greater due to the Mn layer of 20 nm in thickness on the CoCrPt layer, causing a low signal to noise ratio (SNR). In addition, the magnetic head floats less stably since the magnetic layer surface becomes more uneven with an increasing layer thickness.
As described above, a higher recording density would hardly be provided to magnetic recording media, if one wanted to manufacture the magnetic recording media with excellent productivity by any of the conventional techniques. Thus, there is a need for a magnetic recording medium manufacturing technique that allows greater productivity, while producing a product that exhibits a high recording density. The present invention addresses this need.